There are a wide variety of multi-component coating compositions available for finishing automobiles and trucks. Various coating compositions comprising anhydride or epoxy containing compositions are known. For example, U.S. Pat. No. 4,906,677 discloses a composition comprising an acrylic anhydride polymer, a glycidyl component, and a phosphonium catalyst. U.S. Pat. No. 3,136,736 and British patent 994,881 disclose coating compositions comprising polyepoxides and maleic anhydride copolymers. U.S. Pat. No. 4,732,791 concerns a coating composition comprising a polyepoxide, a monomeric anhydride curing agent, and a hydroxyl containing polyfunctional polymer.
U.S. Pat. No. 4,906,677, in column 4, lines 40-47, discloses that anhydride polymers, in epoxy-anhydride compositions, may be converted to a half-ester by alcohol solvents.
European Patent Application, publication No. 0 353 734 A2, published 07.02.90 and European Patent Application, publication No. 0 450 963 A1, published 09.10.91 both disclose the combination of a polymer having a half-esterified acid anhydride group and a compound having an epoxy group and a hydroxy group, or a separate epoxy and hydroxy compound. These patents teach that, in such compositions, curing results from the half-esterified acid anhydride groups being ring-closed at curing temperatures to produce acid anhydride groups which then react with hydroxyl groups, which in turn release carboxyl groups which then react with the epoxy groups. These patents indicate that a certain hydroxy equivalent number or hydroxy value is necessary for adequate water resistance of the coating composition.
The afore-mentioned EP 0 353 734 A2 states that water can be used as a "diluent" if the carboxyl groups are neutralized with an amine. In contrast, the present invention is directed to compositions in which water is employed as the primary carrier, organic solvents being possible diluents. The afore-mentioned EP 0 450 963 A1 states that the half esterified polymer may be neutralized with amines to make a hydrophilic polymer which can form an aqueous composition. The examples, however, all disclose compositions which employ organic solvents.
A problem with present coating compositions for automobiles and trucks, or parts thereof, is that durability is not as good as desired. An important aspect of durability is environmental etch resistance. The present invention offers a high quality finish exhibiting superior environmental resistance at lower cost. In particular, the present invention is directed to an etch-resistant waterborne finish which is dramatically superior in this regard to conventional waterborne finishes.
The present composition is a two-package system, having use in both original equipment manufacture. The half-ester is converted back to an anhydride, for reaction with the epoxy crosslinker during baking and curing at the elevated temperatures at which cars, trucks, parts thereof, and the like are finished. Such a coating composition exhibits excellent clarity, appearance and environmental resistance. Importantly, such compositions offer a low VOC (volatile organic content) since the binder is waterborne employing an aqueous (primarily water) carrier.